Yes, it'll be developed as a openkm plugin ( but depending your country it could be some diference ). Althought it'll be done as OpenKM 5.0. plugin it'll not comes with default intallation ( we're actually studying with which plugin will comes 5.0 and which it'll needed to buy, etc... )

Never mind, I found that the Digital Signature Toolkit works in Windows (on the client level) to upload and attach a digital signature to the pdf directly to the repository (which is on a Linux vm). Not quiet what I was expecting, but should be sufficient.

Digital signature tools comes only for windows. There are another options for digital signature but needs all of them some kind of customization. That's the reason why we have only released a windows version what covers almost cases, however there're other services like automatic signature, safe repository based in long term signatures system or sign server integration ( based in rest ) among other options like biometric signature or thirdparty integrations.

Why do you think the second signature invalidates the first ?
Explain the process how you are signing ( step by step, from the first until the second, with small description what help us imagine how the user is doing it ).

I can add a doc to the OpenKM Digital Signature Client and apply a signature just as it is intended. Open the doc shows that signature is applied and valid.

Upon adding a second signature (by a second user) through the OpenKM Digital Signature Client all goes well and the document is updated in OpenKM.

However when I download that doc and look at the signatures, only the last signature is valid. When I try to look at the previous signatures in the Signature Panel it says that the document has been corrupted.

I have since developed a workflow without the OpenKM Digital Signature Client, but I will reproduce the error and try to post up some screen captures.

If it's possible you share with us the 3 documents ( not signed, with one signature and with two signature will try to review it ). Also do some screenshots for understanding how are you doing the second signature and will try to reproduce in the same way.